The present invention relates to navigation systems. In particular, a receiver for correlating ranging signals at different frequencies uses a common structure.
The Global Positioning System (GPS) is a satellite based navigation system having a constellation of 24 Earth orbiting satellites. These satellites are approximately uniformly dispersed around six circular orbits having four satellites each. Theoretically, four or more GPS satellites are visible from most points on the Earth's surface.
Each GPS satellite presently transmits at two frequencies: L1 (1575.42 MHz) and L2 (1227.60 MHz). There exists provision (for the future) for a third frequency L5 (1176.45 MHz) as well. The L1 frequency has two different spread-spectrum codes modulated on it: a coarse acquisition (C/A) code and a Y code. The C/A code is an unclassified code intended for civilian navigation. It has a chipping rate of 1.023 MHz and a sequence length of 1023 chips. The Y code is a classified unknown code than includes a P code. Both C/A and P codes are unique for each satellite.
GPS receivers are commonly used for a variety of applications involving tracking of the position of various objects. The object to be tracked is coupled to one or more GPS antennas that receive signals from the GPS satellites. A commonly used method that yields position information (within meters) is the pseudorange method. This method utilizes the C/A code and/or the P code modulated onto the carrier signals from the GPS satellites. In another method for more accurate positioning, a reference antenna employs carrier phase measurements and has known coordinates. Differential carrier phase GPS measurement is a technique which determines the position of a given antenna with respect to the reference antenna. The other antennas, known as roving antennas, are free to roam around. Measurements of the carrier phase at the reference antenna and the roving antennas are used to calculate the relative position of the antennas to centimeter level accuracy.
To determine position accurately, the carrier cycle ambiguity or the number of complete carrier cycles between the antennas (reference antenna and roving antennas) is determined. Dual frequency receivers that utilize both L1 and L2 frequency signals can determine carrier cycle ambiguities faster than a single frequency receiver. The phase of the L2 carrier is used to assist in resolving the carrier cycle ambiguity of the L1 signals.
Typically, GPS receivers employ dedicated RF sections for both the L1 and the L2 frequencies for every antenna to be tracked. The RF sections down convert respective L1 and/or L2 RF signals and sample the signals for further processing. However, each additional dedicated RF section adds costs. For a receiver operable with L5 signals, additional cost is added. Tracking L1, L2 and L5 may result in use of correlators with increased power as compared to an L1 and L2 receiver.